Quantum cascade lasers (hereafter referred to as “QCLs”) are promising light sources capable of generating electromagnetic waves from the infrared range to the terahertz range (300 GHz to 10 THz) with high power, and the research and development of such quantum cascade lasers has been accelerated in recent years. In particular, there are no promising light sources (small light sources formed of a compound semiconductor) capable of generating electromagnetic waves in the terahertz range, except for QCLs. Therefore, QCLs are the most promising light sources capable of generating electromagnetic waves in the terahertz range. In such QCLs, intersubband population inversion in the conduction band or intersubband population inversion in the valence band in a multiple quantum well (hereafter referred to as “MQW”) structure formed of a compound semiconductor occurs, which causes lasing.
QCLs capable of generating electromagnetic waves in the terahertz range have been often produced using a GaAs-based material, an InP-based material, or a GaSb-based material. However, lasing at a temperature of 200 K or higher has not been reported with any of the materials.
The factor of inhibiting lasing at a temperature of 200 K or higher is thermally excited phonon scattering. The thermally excited phonon scattering refers to a phenomenon in which when electrons or holes (hereafter referred to as “carriers”) at the upper lasing level obtain in-plane kinetic energy by heat and thus the difference in energy between the carriers and the lower lasing level is higher than or equal to the vibrational energy of longitudinal optical (hereafter referred to as “LO”) phonons, the carriers are scattered by the LO phonons to cause nonradiative relaxation to the lower lasing level.
For example, in a QCL (including a GaAs layer as a quantum well layer) that generates electromagnetic waves with about 2 THz (about 9 meV), the energy difference between the upper lasing level and the lower lasing level is about 9 meV (energy corresponding to lasing wavelength). At low temperatures, carriers are often present at the bottom of the energy band of the upper lasing level. Therefore, the transition of carriers present at the upper lasing level to the lower lasing level by LO phonon scattering does not occur.
However, when the temperature increases, carriers are thermally excited and the distribution becomes similar to, for example, a quasi-Fermi distribution. When the difference in energy between carriers thermally excited to the level higher than the bottom of the energy band of the upper lasing level and the lower lasing level is concordant with the vibrational energy of LO phonons of a compound semiconductor constituting the quantum well layer of the QCL, the thermally excited carriers are subjected to transition to the lower lasing level by LO phonon scattering. This transition occurs with higher probability than the stimulated emission from the upper lasing level to the lower lasing level. That is, the thermally excited carriers lose energy by not generation of electromagnetic waves but by LO phonon scattering. In addition, if the thermally excited carriers are subjected to transition to the lower lasing level by LO phonon scattering, the carriers that have lost energy occupy the lower lasing level, which suppresses the occurrence of population inversion. As a result, lasing is suppressed.
For example, in the above-described QCL, when carriers present at the upper lasing level obtain an energy of about 27 meV by heat, LO phonon scattering (nonradiative transition) becomes dominant, which suppresses lasing. Herein, the vibrational energy of LO phonons is a physical property intrinsic to materials. Therefore, if a GaAs layer is used as a quantum well layer in a QCL capable of generating electromagnetic waves in the terahertz range, it is difficult to operate the QCL at room temperature.
It is proposed in PTL 1 that a layer made of a compound semiconductor different from GaAs is used as a quantum well layer in a QCL capable of generating electromagnetic waves in the terahertz range. PTL 1 discloses that a GaN layer is used as a quantum well layer, and the vibrational energy of LO phonons of GaN is about 90 meV. Therefore, the occurrence of the above-described thermally excited phonon scattering is believed to be prevented.